1. Field of the Disclosure
The present disclosure relates to a method and an apparatus for performing a cell search and a frequency offset estimation in a communication system.
2. Description of the Related Art
A general cell search process of a receiver that is used in a communication system includes three steps. The first step involves slot boundary identification using a primary synchronization channel (P-SCH). The second step involves frame boundary and code group identification using a secondary synchronization channel (S-SCH). The third step involves scrambling code identification using a primary common pilot channel (P-CPICH). A step that involves a multipath search using the P-CPICH may be added for timing configuration by a demodulator.
The above-described steps for the cell search process determine whether information required for the respective steps is detected using a correlation value between a received signal and a specific progression. Due to the existence of a frequency offset in a general wireless communication environment, a phase of the received signal rotates and the correlation value deteriorates, thereby reducing the effectiveness of the cell search.
When the frequency offset exists, if a coherent accumulation length is configured to be small, an influence of the frequency offset on the correlation value may be reduced. However, this also results in a reduction in a noise suppression effect through the synchronous accumulation. Further, although cell search performance may be more effective, information on the frequency offset cannot be transmitted to the demodulator, thereby reducing a demodulation success rate.
In another method, after scrambling code identification in the third step of the cell search process, fast fourier transform (FFT) is performed on a correlation value between a despread result of the received signal and a CPICH pattern in every slot. An index of the maximum value generated by curve fitting values around the largest value of the FFT outputs is derived as a frequency offset. Further, a cell search success/failure is determined by comparing an FFT output-based metric with a predetermined threshold value. This method can increase the effectiveness of the cell search, but also increases implementation complexity due to the use of a separate FFT device. Further, since the FFT and curve fitting operations are required to be performed in every search, power consumption and cell search time increase.
As described above, when the frequency offset exists, the conventional technologies can improve the effectiveness of the cell search, but do not consider that frequency offset estimation for improving demodulation capability should be performed after the cell search, or that such an estimation increases implementation complexity, power consumption, and cell search time.